This application is based on Japanese Patent Application Nos. 11-266614 filed Sep. 21, 1999 and 11-337750 filed Nov. 29, 1999, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a brake control apparatus for controlling braking forces to be generated by a plurality of brake devices for braking respective wheels of a vehicle.
2. Discussion of Related Art
An example of such a brake control apparatus is disclosed in JP-B2-2835173. This brake control apparatus is adapted to control braking forces to be generated by respective brake devices in the form of four wheel brakes for braking four wheels of an automotive vehicle, namely, a front left wheel, a front right wheel, a rear left wheel and a rear right wheel of the vehicle. The brake control apparatus includes a brake-operation inhibiting portion operable when at least one of the wheel brakes is defective, for inhibiting an operation of one of the normal wheel brakes which is determined by the location of the at least one wheel corresponding to the defective at least one wheel brake.
Where the wheel brake for the rear right wheel is defective, for instance, the brake-operating inhibiting portion of the brake control apparatus indicated above inhibits an operation of the normal wheel brake for the rear left wheel. Where the wheel brakes for the front left and right wheels are defective, the brake-operation inhibiting portion inhibits an operation of the normal wheel brake for the rear right wheel. This control arrangement reduces a difference between the total braking force generated by the two wheel brakes on the left side of the vehicle and the total braking force generated by the two other wheel brakes on the right side, so that the running stability of the vehicle is improved.
The present inventors have found that the known brake control apparatus indicated above has a further room for improvement for more effectively utilizing the normal wheel brake or brakes while restraining a reduction in the running stability of the vehicle when at least one of the wheel brakes is defective. That is, the inventors have found that there are some cases where a reduction in the running stability of the vehicle due to a difference between the total braking force of the left wheel brakes and that of the right wheel brakes is not so large as to cause a problem.
It is therefore an object of the present invention to provide a brake control apparatus which permits effective utilization of the normal brake devices while assuring a substantially acceptable degree of running stability of the vehicle.
The above object may be achieved may be achieved according to any one of the following modes of the present invention, each of which is numbered like the appended claims and depends from the other mode or modes, where appropriate, to indicate and clarify possible combinations of elements or technical features. It is to be understood that the present invention is not limited to the technical features or any combinations thereof which will be described for illustrative purpose only. It is to be further understood that a plurality of elements or features included in any one of the following modes of the invention are not necessarily provided all together, and that the invention may be embodied without some of the elements or features described with respect to the same mode.
(1) A brake control apparatus for controlling at least three brake devices provided for braking respective at least three wheels of an automotive vehicle which includes two wheels located on respective left and right sides of the vehicle, the brake control apparatus comprising an emergency brake control portion for controlling at least two normal brake devices of the at least three brake devices when at least one of the at least three brake devices is defective, such that a difference between a total left-side braking force to be generated by at least one of the at least two normal brake devices which is located on the left side of the vehicle and a total right-side braking force to be generated by the other of the at least two normal brake devices which is located on the right side of the vehicle is made larger when operations of all of the at least two normal brake devices in a detected running condition of the vehicle are not likely to deteriorate a running stability of the vehicle, than when the operations of all of the at least two normal brake devices in the detected running condition are likely to deteriorate the running stability of the vehicle.
In the brake control apparatus constructed according to the above mode (1) of this invention, the emergency brake control portion is operable when at least one of the at least three brake devices, for controlling at least two normal brake devices of the at lest three brake devices, such that a difference between the total left-side braking force to be generated by at least one of the at least two normal brake devices which is located on the left side of the vehicle and the total right-side braking force to be generated by the other of the at least two normal brake devices which is located on the right side of the vehicle is made larger when operations of all of the at least two normal brake devices in the detected vehicle running condition are not likely to deteriorate the vehicle running stability, than when the operations of all of the at least two normal brake devices in the detected vehicle running condition are likely to deteriorate the vehicle running stability.
In the known brake control apparatus, when the brake device for the rear right wheel is defective, the operation of the normal brake device for the rear left wheel is necessarily inhibited. To the contrary, the present brake control apparatus may be adapted to permit the operation of the normal brake device for the rear left wheel when the operation of this normal brake device for the rear left wheel in the presently detected vehicle running condition is not likely to deteriorate the vehicle running stability. Thus, the present brake control apparatus permits effective utilization of the normal brake device for the rear left wheel, thereby reducing the reduction of the total braking force of the vehicle when at least one of the brake devices is defective.
(2) A brake control apparatus according to the above mode (1), wherein the emergency brake control portion includes a running-condition detecting portion for effecting a determination as whether the operations of all of the at least two normal brake devices in the detected running condition of the vehicle are likely to deteriorate the running stability of the vehicle or not, on the basis of at least one of a detected running speed and a detected steering state of the vehicle.
Where the defective brake device is provided for the wheel which is located on the inner side of the path of turning of the vehicle, for instance, the operations of all the normal brake devices in the detected vehicle running condition including. the detected vehicle steering state (steering angle and direction) are less likely to deteriorate the vehicle running stability than when the defective brake device is provided for the wheel located on the outer side of the path of turning of the vehicle. When the vehicle is turning with the defective brake device located on the inner side of the vehicle turning path, a moment acts on the vehicle in the turning direction, while a moment due to a difference between the total inner-side braking force and the total outer-side braking force acts on the vehicle in the direction opposite to the turning direction of the vehicle, since the total outer-side braking force is larger than the total inner-side braking force on the inner side. Accordingly, the vehicle has a comparatively low spinning tendency owing to an effect of offsetting of the moment due to the turning of the vehicle and the moment due to the braking force difference on the inner and outer sides of the vehicle turning path when the defective brake device is located on the inner side of the vehicle turning path. Where the defective brake device is located on the outer side of the vehicle turning path, on the other hand, the moment due to the vehicle turning and the moment due to the braking force difference act on the vehicle in the same direction, namely, in the vehicle turning direction, so that the vehicle has a comparatively high spinning tendency. Thus, the vehicle has a higher degree of vehicle running stability where the defective brake device is located on the inner side of the vehicle turning path.
Where the defective brake device is located on the outer side of the vehicle turning path, the vehicle running stability is considered to be higher when the steering angle of the vehicle (steering angle of the front wheels) is relatively small than when the steering angle is relatively large. For example, the vehicle is considered to have a comparatively high degree of running stability when the detected steering angle is smaller than a predetermined threshold. The determination as to whether the operations of all of the normal brake devices are like to deteriorate the vehicle running stability may be effected on the basis of the steering angle of the steering wheel, vehicle yaw rate or other parameters, other than the steering angle of the front wheels.
Further, the vehicle running stability is considered to be higher when the vehicle running speed is relatively low than when the vehicle running speed is relatively high. For instance, the vehicle is considered to have a comparatively high degree of running stability when the detected vehicle running speed is lower than a predetermined threshold. When the vehicle running speed is relatively low, the lateral force expected to act on the vehicle at a given steering angle of the vehicle is smaller than when the vehicle running speed is relatively high. Accordingly, the moment due to the braking force difference on the left and right sides of the vehicle will not have a considerable influence on the running behavior of the vehicle. Even if this moment more or less deteriorate the vehicle running stability, the deteriorated running stability may be comparatively easily rectified by the vehicle operator by manipulating the steering wheel.
It is also noted that the determination as to whether the operations of all of the normal brake devices are likely to deteriorate the vehicle running stability or not may be accomplished with a higher degree of accuracy, on the basis of both of the steering state and the running speed of the vehicle.
(3) A brake control apparatus according to the above mode (2), wherein the running-condition detecting portion determines that the operations of all of the at least two normal brake devices in the detected running condition of the vehicle are not likely to deteriorate the running stability of the vehicle, when the detected running speed is lower than a predetermined threshold determined by the detected steering state of the vehicle.
In the brake control apparatus according to the above mode (3), the threshold of the vehicle running speed may be determined to be lower when the detected steering angle of the vehicle is relatively large than when the detected steering angle is relatively small. It is reasonable to determine that the vehicle has a relatively high degree of running stability when the steering angle is relatively small even where the vehicle running speed is relatively high, or when the vehicle running speed is relatively low even where the steering angle is relatively large.
(4) A brake control apparatus according to any one of the above modes (1)-(3), wherein the emergency brake control portion includes a turning-direction-based running-condition detecting portion for detecting a determination as to whether the operations of all of the at least two normal brake devices in the detected running condition of the vehicle are likely to deteriorate the running stability of the vehicle, on the basis of a location of each of said at least one defective brake device on the vehicle and a detected direction of turning of the vehicle.
(5) A brake control apparatus according to the above mode (4), wherein the turning-direction-based running-condition detecting portion determines that the operations of all of the at least two normal brake devices in the detected running condition are not likely to deteriorate the running stability of the vehicle when said at least one defective broke device includes a brake device provided for one of the at least three wheels which is located on an inner side of a path of turning of the vehicle in the detected direction.
(6) A brake control apparatus according to any one of the above modes (1)-(5), wherein the emergency brake control portion determines that the vehicle has a higher degree of running stability when the number of said at least one defective brake device is relatively small than when the above-indicated number is relatively large.
The difference between the total left-side braking force to be generated by all of the at least one normal brake device located on the left side of the vehicle and the total right-side braking force to be generated by all of the at least one normal brake device located on the right side of the vehicle is larger where the at least one defective brake device consists of two brake devices located on one of the left and right sides of the vehicle, than where only one brake device on one side of the vehicle is defective. Where the two brake devices on one side of the vehicle are defective, the operations of the normal brake devices located on the other side of the vehicle may cause a considerably large braking force difference on the left and right sides of the vehicle, leading to a risk of deterioration of the driving stability of the vehicle. Thus, it is reasonable to determine that the vehicle has a lower degree of running stability when two brake devices are defective than when only one brake device is defective. In other words, it is reasonable to determine that the vehicle has a higher degree of running stability and the normal brake devices can be more effectively utilized, when only one brake device is defective.
(7) A brake control apparatus according to any one of the above modes (1)-(6), wherein the emergency brake control portion controls a braking force to be generated by each of at least one of the at least two normal brake devices, on the basis of the detected running condition of the vehicle.
In the brake control apparatus according to the above mode (7) wherein the braking force to be generated by at least one of the at least two normal brake devices is controlled on the basis of the detected vehicle running condition, it is possible to reduce the amount of reduction of the total vehicle braking force due to the defect of the at least one brake device, while maintaining the vehicle running stability at a substantially acceptable level.
For instance, the braking force of the normal brake device may be controlled to a value corresponding to the detected degree of vehicle running stability. The braking force to be generated may be controlled such that it is increased with an increase in the vehicle running stability. The controlled braking force may be increased either continuously or in steps as the detected vehicle running stability increases. The controlled braking force may be zeroed as needed. Namely, the operation of at least one of the normal brake devices may be inhibited. The at least one normal brake whose braking force is controlled preferably includes at least one brake device which is located on one side of the vehicle which is opposite to the side on which at least one of the at least one defective brake device is located.
For example, the braking force of the normal brake device may be controlled on the basis of the detected running speed of the vehicle. The vehicle running stability is considered to be reduced with an increase in the vehicle running speed, so that the braking force to be generated by the normal brake device is reduced with an increase in the vehicle running speed. Where the braking force to be generated the normal brake device is controlled to a value larger than zero but smaller than the normally desired value, the total vehicle braking force can be made larger than where the operation of the normal brake device is inhibited, and the vehicle running, stability can be higher than where the braking force is controlled to the normally desired value.
Alternatively, the braking force of the normal brake device may be controlled on the basis of the detected steering state of the vehicle. For instance the braking force may be reduced with an increase in the steering angle of the front wheels, since the vehicle running stability is reduced with an increase in the steering angle of the vehicle.
(8) A brake control apparatus according to any one of the above modes (1-(7), wherein the emergency brake control portion includes a change-rate restricting portion for controlling at least one of a rate of increase and a rate of reduction of a braking force to be generated by each of at least one of the at least two normal brake devices, such that each of the at least one of the rates of increase and reduction is lower when at least one of the at least three brake devices is found defective than when none of the at least three brake devices is found detective.
In the brake control apparatus according to the above mode (8) wherein the rate of change of the braking force to be generated by the normal brake device is made lower when at least one of the at least three brake devices is detective, the rate of change of the braking force difference on the left and right sides of the vehicle can be reduced. That is, the rate of change of the braking force to be generated by the normal brake device is restricted so as to prevent an excessively high rate of change of the braking force difference, so that the rate of change of the moment acting on the vehicle due to the braking force difference is restricted. Accordingly, the vehicle running stability which is more or less deteriorated due to a change in the moment acting on the vehicle can be rectified by the vehicle operator by manipulating the steering wheel.
The above-indicated restriction of at least one of the rates of increase and reduction of the braking force may be effected for all of the at least two normal brake devices, or for at least one of the normal brake device which is located on one side of the vehicle that is opposite to the side on which at least one of the at least one defective brake device is located.
The rate of increase and/or the rate of reduction of the braking force may be restricted to a predetermined constant value or a value which varies with the detected running condition of the vehicle.
(9) A brake control apparatus according to any one of the above modes (1)-(7), wherein the emergency brake control portion includes a change-rate restricting portion for controlling at least one of a rate of increase and a rate of reduction of a braking force to be generated by each of at least one of the at least two normal brake devices, such that each of the at least one of the rates of increase and reduction is lower when normal operations of all of the at least two normal brake devices in the detecting running condition of the vehicle are likely to increase the difference between the total left-side braking force and the total right-side braking force, than when the normal operations are not like to increase the difference.
Where the normal operations of all of the normal brake devices are likely to increase the braking force difference on the left and right sides of the vehicle, it is preferable to restrict the rate of change of the braking force to be generated by at least one of the normal brake devices. When the normal operations of the normal brake devices are not likely to considerably increase the braking force difference, it is not necessary to restrict the rate of change of the braking force to be generated by the normal brake device or devices.
(10) A brake control apparatus for controlling a plurality of brake devices for braking respective wheels of an automotive vehicle, wherein a change-rate restricting portion is provided for restricting at least one of a rate of increase and rate of reduction of a barking force to be generated by each of at least one of at least one normal brake device of the plurality of brake devices, when at least one of the plurality of brake devices is found defective, such that each of the at least one of the rates of increase and reduction is lower when the at least one of the at least three brake devices is found defective than when none of the plurality of brake devices is found defective.
(11) A brake control apparatus according to any one of the above modes (1)-(10, wherein each of the brake devices includes a friction member movably supported by a body of the automotive vehicle, and an electrically operated actuator operable to force the friction member against a rotor rotating with a corresponding one of the wheels, the electrically operated actuator being operated with an electric energy to be supplied thereto such that a force by which the friction member is forced against the rotor by the actuator is controllable by controlling an amount of the electric energy.
Each of the brake devices may be an electrically operated brake device including an electrically operated actuator in the form of an electric motor, for instance. In this type of electrically operated brake device, the friction member is moved by the electric motor other electrically operated actuator, into pressing contact with the rotor rotating with the corresponding wheel, so that the wheel is braked. The pressing force by which the friction member is forced against the rotor is controllable by controlling the amount of the electric energy to be supplied to the electrically operated actuator.
Alternatively, each of the brake device may be a hydraulically operated brake device including a wheel brake cylinder, and a hydraulic pressure control device capable of regulating the pressure of a fluid in the wheel brake cylinder.
The hydraulic pressure control device includes at least one solenoid-operated control valve disposed in a hydraulic circuit connected to the wheel brake cylinder, a high-pressure source and a low-pressure source. By controlling the at least one control valve, the fluid pressure in the wheel brake cylinder can be increased and reduced, for thereby controlling the force by which the friction member is forced against the rotor. In this case, the wheel brake cylinder and the hydraulic pressure control device constitute an electrically controlled actuator. In a further alternative arrangement, the electrically controlled actuator includes a wheel brake cylinder, a pump connected to the wheel brake cylinder, an electrically operated pump motor for driving the pump. The pressure of the pressurized working fluid delivered from the pump is controlled by controlling the pump motor, so that the fluid pressure in the wheel brake cylinder is accordingly controlled. In this type of electrically controlled actuator, the pump motor, wheel brake cylinder and working fluid may be considered as a drive source, an output member, and a power transmitting medium, respectively.
In the brake device of any one of the types indicated above, the braking force to be generated by the brake device is controllable by controlling the amount of electric energy supplied to the electrically operated and controlled actuator. The braking force may be controlled to a value corresponding to an amount of operation of a brake pedal or other brake operating member, or to a value not corresponding to the amount of operation of the brake operating member. Since the brake device generates a braking force by application of an electric energy to the electrically controlled actuator, the brake device fails to generate a braking force even with the electrically controlled actuator being supplied with the electric energy, when the actuator is defective. The brake device may be arranged to mechanically generate a braking device based on the operating force acting on the brake operating member. In this case, however, the brake device is required to generate not only the mechanically generated braking force, but also a braking force which varies with a change in the controlled amount of electric energy to be supplied to the defective actuator.
The brake device is not limited to the friction type indicated above, but may be a regenerative brake device which utilizes a wheel drive electric motor operable to generate a regenerative braking effect for braking the corresponding wheel. In this case, the electric motor is provided for each of the wheels, for driving the corresponding wheel, and is controllable to control the regenerative braking torque to be applied to the corresponding wheel.
(12) A brake control apparatus for controlling a plurality of brake devices for braking respective wheels of an automotive vehicle, wherein an emergency brake control portion is provided for controlling at least one normal brake device of the plurality of brake devices when at least one of the plurality of brake device is defective, such that a braking force to be generated by each of at least one of the at least one normal brake device is controlled on the basis of a detected running speed of the vehicle.
In the brake control apparatus according to the above mode (12), the braking force to be generated by each of at least one of the at least one normal brake device is controlled by the emergency brake control portion, on the basis of the detected vehicle running speed.
The brake control apparatus according to the above mode may include the technical feature according to any one of the above modes (1)-(11).
(13) A brake control apparatus according to the above mode (12), wherein the emergency brake control portion operates all of the at least one normal brake device when the detected running speed of the vehicle is lower than a predetermined threshold, and inhibits an operation of at least one of the at least one normal brake device when the detected running speed is not lower than the predetermined threshold.
When the vehicle running speed is lower than the predetermined threshold, the vehicle running stability can be maintained at a substantially acceptable level even if all of the at least one normal brake device are operated. In this condition, the at least one normal brake device can be effectively utilized when at least one of the brake devices is defective. When the vehicle running speed is not lower than the threshold, it is desirable to inhibit the operation of at least one of the at least one normal brake device, which is determined by the location and the number of the at least one defective brake device. For instance, it is desirable to inhibit the operation of the normal brake device located on one side of the vehicle opposite to the side on which at least one of the at least one defective brake device is located. In this case, the braking force difference on the opposite sides of the vehicle can be reduced, and the amount of reduction of the vehicle running stability due to the defect of at least one brake to device can be reduced.
(14) A brake control apparatus according to the above mode (12) or (13), wherein the emergency brake control portion includes a first braking-force restricting portion for controlling a braking force to be generated by each of the above-indicated at least one of the at least one normal brake device such that the braking force is smaller when the detected running speed of the vehicle is higher than a predetermined threshold than when the detected running speed is not higher than the predetermined threshold.
(15) A brake control apparatus according to any one of the above modes (13) or (14), wherein the emergency brake control portion includes a second braking-force restricting portion for determining the threshold value such that the threshold value is smaller when the number of the at least one defective brake device is relatively large than when the number is relatively small.
(16) A brake control apparatus for controlling a plurality of brake devices for braking respective wheels of an automotive vehicle, wherein an emergency brake control portion is provided for controlling at least one normal brake device of said plurality of brake devices when at least one of the plurality of brake devices is defective, such that a braking force to be generated by each of at least one of the at least one normal brake device is controlled on the basis of a detected steering state of the vehicle.
In the brake control apparatus according to the above mode (16) wherein the braking force of each of at least one of the at least one normal brake device is controlled on the basis of the detected steering state of the vehicle, it is possible to reduce the amount of reduction of the total vehicle braking force due to the defect of the at least one defective brake device, while maintaining the vehicle running stability at a substantially acceptable level.
The brake control apparatus according to the above mode (16) may include the technical feature according to any one of the above modes (1)-(15).
(17) A brake control apparatus according to the above mode (16), wherein the emergency brake control portion includes a third braking-force restricting portion for controlling at least one of the at least one normal brake device which is located on one of opposite sides of the vehicle which is opposite to the other side on which one of the at least one defective brake device is located, such that a braking force to be generated by each of the at least one of the at least one normal brake device located on the above-indicated one of opposite sides of the vehicle is made smaller when the above-indicated one of the at least one defective brake device which is located on the above-indicated other side of the vehicle is located on an outer side of a path of turning of the vehicle than when the above-indicated one of the at least one defective vehicle is located on an inner side of the path of turning of the vehicle.
(18) A brake control apparatus according to the above mode (17), wherein the third braking-force restricting portion controls the braking force to be generated by the each of the at least one of the at least one normal brake device such that the braking force is smaller when a detected steering angle of the vehicle is relatively large than when the detected steering angle is relatively small.
(19) A brake control apparatus according to any one of the above modes (16)-(18), wherein the emergency brake control portion further includes a running-speed-based brake control portion for controlling the braking force to be generated by each of the above-indicated at least one of the at least one normal brake device, on the basis of a detected running speed of the vehicle.
In the brake control apparatus according to the above mode (19) wherein the braking force of the selected normal brake device or devices is controlled on the basis of the detected vehicle running speed as well as the detected vehicle steering state, the reduction of the running stability of the vehicle due to the at least one defective brake device can be further reduced.
(20) A brake control apparatus for controlling a plurality of primary brake devices for braking respective wheels of an automotive vehicle, and at least one secondary brake device provided for at least one of said wheels, respectively, said brake control apparatus comprising an emergency brake control portion operable when at least one of the plurality of primary brake devices is defective, for operating at least one of the at least one secondary brake device which corresponds to at least one of the at least one defective primary brake device.
In the brake control apparatus according to the above mode (20) wherein the secondary brake device corresponding a defective one of the primary brake device is operated, the amount of reduction of the total vehicle braking force due to the defective primary brake device can be reduced, and the braking force difference on the opposite sides of the vehicle can be reduced so as to reduce the deterioration of the vehicle running stability, than where the secondary brake device is not operated.
For instance, the primary brake devices are used as service brake devices while the secondary brake device or devices is/are used as a parking brake device or devices. One and the other of the primary brake device and the secondary brake device which are provided for the same wheel may include a disc brake and a drum brake, respectively. Where wheel drive motors are provided for the respective wheels, one and the other of the primary and secondary brake devices for the same wheel may include a regenerative brake and a disc brake, respectively.
The brake control apparatus according to the above mode (20) may include the technical feature according to any one of the above modes (1)-(19).
(21) A brake control apparatus according to the above mode (20), wherein the emergency brake control portion includes an emergency primary-braking-force control portion for controlling a braking force to be generated by each of at least one normal primary brake device of the plurality of primary brake devices, so that a running stability of the vehicle as represented by a detected running condition of the vehicle is higher than a predetermined threshold.
In the brake control apparatus according to the above mode (20) wherein the braking force of at least one normal primary brake device is controlled as well as the secondary brake device corresponding to the defective primary brake device is operated, the running stability of the vehicle can be maintained at a relatively high level.
(22) A brake control apparatus according to the above mode (21), wherein the emergency primary-braking-force control portion controls the braking force to be generated by each of,the above-indicated at least one normal primary brake device, on the basis of the detected running condition of the vehicle.
(23) A brake control apparatus according to any one of the above modes (20)-(22), wherein the emergency brake control portion includes a braking-force-difference reducing type brake control portion for controlling at least one of braking forces to be generated by the primary and secondary brake devices which are normal, such that a difference between a total left-side braking force to be generated by all of at least one of the primary and secondary brake devices that is located on a left side of the vehicle and a total right-side braking force to be generated by all of the other of the primary and secondary brake devices that is located on a right side of the vehicle will not exceed than a predetermined threshold.
Where at least one of the plurality of primary brake devices is defective, the braking force difference on the left and right sides of the vehicle can be reduced by controlling at least one of the braking forces to be generated by the primary and secondary brake devices.
In the case where the maximum braking forces that can be generated by the primary and secondary brake devices are different from each other, for instance, where the maximum braking force of each secondary brake device is smaller that that of each primary brake device, the operation of the secondary brake device corresponding to the defective primary brake device may still cause a difference between the total braking forces on the left and right sides of the vehicle. In view of this fact, the emergency brake control portion according to the above mode (23) includes the braking-force-difference reducing type control portion adapted to control the braking force to be generated by an appropriate one or ones of the normal primary and secondary brake devices, for example, the braking force to be generated by at least one of the normal primary brake devices, so as to reduce the braking force difference on the left and right sides of the vehicle. Accordingly, the reduction of the vehicle running stability due to the defective primary brake device or devices can be reduced. The braking-force-difference reducing type control portion may be adapted to operate the selected one or ones of the normal primary and secondary brake devices with a suitable duty ratio, namely, alternately turn on and off the brake device or devices such that the predetermined braking force is generated for a predetermined first length of time and is zeroed for a predetermined second length of time.
A determination as to whether the difference between the total left and right braking forces indicated above is larger than the predetermined threshold or not may be effected on the basis of the actually detected braking forces generated by all of the normal brake devices, or alternatively on the basis of the detected steering state of the vehicle. The braking forces can be directly detected on the basis of the force by which the friction member is forced against the wheel rotor, for instance, or indirectly detected on the basis of the commanded braking force value. The steering state of the vehicle can be represented by the detected yaw rate of the vehicle or the steering angle of the front wheels or steering wheel. If the detected yaw rate or steering angle is higher or larger than a predetermined upper limit, the braking force difference is considered to be larger than the predetermined threshold.
In a braking system wherein four sets of a primary brake device and a secondary brake device are provided for braking respective front left, front right, rear left and rear right wheels, the two primary brake devices for the front and rear wheels on one of opposite sides of the vehicle may be defective. In this case, the secondary brake devices for those front and rear wheels on the same side of the vehicle may be operated. In this control arrangement, however, there arises a considerably large difference between the total braking forces on the opposite sides of the vehicle, since the two secondary brakes are operated on one side of the vehicle while the two primary brakes are operated on the other side of the vehicle. To reduce the braking force difference and thereby improve the vehicle running stability, the braking-force-difference reducing type control portion may be adapted to restrict or zero the braking force to be generated by one of the two primary brake devices.
(24) A brake control apparatus according to the above mode (23), wherein the braking-force-difference reducing type control portion includes a primary-braking-force restricting portion for operating all of the at least one secondary brake device that corresponds to all of the at least one defective primary brake device, and restricting the braking force to be generated by each of at least one of normal ones of the primary brake devices.
In the brake control apparatus according to the above mode (24), the secondary brake device corresponding to each defective primary brake device is operated, and the braking force to be generated by at least one normal primary brake device is restricted when the operation of the secondary brake device or devices is not likely to reduce the braking force difference on the left and right sides of the vehicle to a value smaller than the predetermined threshold. Accordingly, the braking force difference can be reduced.
(25) A brake control apparatus according to the above mode (24), wherein the primary-braking-force restricting portion restricts the braking force to be generated by each of the at least one normal primary brake device, when a running stability of the vehicle as represented by a detected running condition of the vehicle is lower than a predetermined threshold.
The brake control apparatus according to the above mode (25) makes it possible to avoid an unnecessary restriction of the braking force of the normal primary brake device or devices.
(26) A brake control apparatus according to any one of the above modes (20)-(25), wherein the emergency brake control portion includes a braking-force reduction restricting control portion for controlling at least one of braking forces to be generated by the primary and second brake devices, so that a total braking force to be applied to the vehicle is not smaller than a desired braking force value as represented by an amount of operation of a brake operating member by an operator of the vehicle.
If the operation of the defective primary brake device is inhibited while the corresponding secondary brake device is operated, the total braking force to be applied to the vehicle may be smaller than the total braking force value desired by the vehicle operator. If the defective primary brake device is not inhibited from being operated but is also commanded to be operated together with the corresponding secondary brake device, the total braking force may be increased owing to a braking force that may be generated by the defective primary brake device
(27) A brake control apparatus according to any one of the above modes (20)-(26), wherein the at least one secondary brake device consists of a plurality of secondary brake devices which are provided for the wheels, respectively, and which correspond to the plurality of primary brake devices, respectively, the primary and secondary brake devices for each of the wheels are connected to respective different power sources.
In the brake control apparatus according to the above mode (27) wherein the primary and second brake devices of each wheel are connected to the different power sources, one of the primary and secondary brake devices can be operated even when one of the power sources which corresponds to the other brake device is defective. Namely, one of the primary and secondary brake devices can be operated with a normal one of the two power sources, to brake the wheel, even when the other power source is defective. The primary and secondary brake devices may include an electrically operated actuator as described above with respect to the above mode (11). In this case, the electrically operated actuators of the primary and secondary brake devices are supplied with an electric energy by the respective different power sources.